Turbocompressors are frequently equipped with a plurality of controllers. The anti-surge controller of a turbocompressor monitors, e.g., the position of the compressor working point in the characteristic diagram and opens an anti-surge control valve to the suction side or the atmosphere in the case of an unacceptably low compressor throughput. To adapt the turbocompressors to the needs of the process side, pressure or flow controllers, whose control units are formed by adjustable guide blades or throttle valves, are frequently used. The speed may also be adjusted for adjusting the capacity in the case of compressors with variable-speed drives.
The adjustment of the anti-surge control valve also influences the compressor discharge pressure and the flow to the process. Adjustment of the control unit of the process variable controller influences the position of the working point in the characteristic diagram and may let the anti-surge controller act as a result.
As a critical turbomachine protective controller, the anti-surge controller is usually set to the fastest possible response behavior. The fastest controllers available, which actuate the fastest valves available, are used for surge line control.
The process variable control must be adapted to the time response of the process. Pressure controls, in particular, are characterized by markedly longer time constants than are necessary for surge line controls. As a result, it is ensured in the normal case that the different control circuits do not interact one with each other in an unacceptable manner. The anti-surge controller corrects a disturbance substantially more rapidly than the process variable controller. It will have brought the surge line control valve into the necessary new position before the process variable controller has responded noticeably. An additional uncoupling of the anti-surge controllers among each other is not necessary in these cases.
However, there are applications in which the anti-surge controller must respond slowly or the process variable controller must respond quickly. The interfere one with each other of the controllers on each other cannot be ruled out in these applications. A disturbance on the compressor suction side may cause, e.g., the working point to move somewhat closer to the surge line.
The anti-surge controller responds to this and opens somewhat the surge line control valve to protect the compressor. As a result, less medium is delivered into the process and the flow (or the pressure) decreases on the delivery side of the compressor. The process variable controller notices this and increases the delivery capacity of the compressor. The consequence of this is that the working point moves away from the surge line. The anti-surge controller now responds to this and closes the surge line control valve correspondingly. However, this allows the pressure as well as the flow to increase on the delivery side of the compressor. The process variable controller responds to this by correspondingly reducing the delivery capacity of the compressor. However, this will again move the working point into the vicinity of the surge line, so that the anti-surge controller will again open the surge line control valve. The process begins anew and may lead to a continuous variation of the process variable and of the surge line control valve if the time parameters are selected unfavorably and the phase position is unfavorable.
Turbocompressors with a plurality of stage groups are protected with individual surge line controls per stage group, especially if side streams or intermediate extractions are used between the different stages. Interfere one with each other of the anti-surge controllers on each other may occur in this case as well. If the pressure ratio is increased over the low pressure stage due to a disturbance on the suction side of the low-pressure stage, the working point of this stage moves in the direction of the surge line, as a result of which an intervention of the anti-surge controller of the low-pressure stage, which opens the surge line control valve of the low-pressure stage somewhat, may become necessary. This causes a reduction in the discharge pressure of the low-pressure stage which is identical to the inlet pressure of the high-pressure stage. This is accompanied by an increase in the pressure ratios of the high-pressure stage, which leads to the opening of the surge line control valve of this stage. Since the surge line control valves release the pressure-side gas toward the suction side in gas compressors, an opening of the high pressure-side surge line control valve causes an increase in the suction pressure of this stage and consequently an increase in the discharge pressure of the low-pressure stage. The surge line control of the low-pressure stage is forced as a result to intervene more, and the low-pressure surge line control valve opens wider.
It is quite possible in the case of rapid transient processes for the anti-surge controller to act more violently than would be absolutely necessary, and the surge line control valve is opened wider than is necessary for the protection of the compressor. The consequence of this is that the surge line control valve will again be closed after the first disturbance has been balanced. Since the disturbance has begun in the low-pressure part, the anti-surge controller of the low-pressure stage will again close this valve. The discharge pressure of this stage thus increases and so does the suction pressure of the high-pressure stage as well. The pressure ratio of the high-pressure stage decreases and the corresponding anti-surge controller closes the high pressure-side surge line control valve. This will again influence the low-pressure part, etc. If the controllers are set such that they respond to a transient disturbance with a certain overmodulation, a phase-shifted interfere one with each other of the two anti-surge controllers on each other cannot be ruled out.
The risk of interactions increases if not only two compressor stage groups, but three or more compressor stages are arranged in series. The process is applicable not only to anti-surge controllers, but in general.